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Topic: Keithley 2100: How does one identify a burnt SMD resistor without service manual (Read 12175 times)

I got my hands on a faulty Keithley 2100 6.5 digit DMM. The problem is that the voltage and resistance readings seem disconnected from the front sockets. Its clear something has gone wrong inside.

After opening the case i identified two burnt SMD resistors. The problem is there are absolutely no service manuals available for this thing! I contacted keithley but they refused to help and insisted i send it in for service.

so...How does one identify a burnt SMD resistor for replacement.

PS: if i have no luck, i am thinking i can short out R321, because R321,R322 and L303 seems like some sort of high frequency filter, the wire left of the inductor is connected to the front banana jacks and the right is going to the digital section.

This looks like a digital output judging by the opto-isolator i guess, perhaps a way to identify this is by following the traces and drawing a small schematic to judge the values from the rest of the circuit, you should start by removing both resistors. BTW i couldnt find a data sheet for U305 (likely an opto isolator)

where does the other end of L303 go?

« Last Edit: August 30, 2016, 09:34:34 pm by h.bashar »

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"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius, and a lot of courage to move in the opposite direction".- Albert Einstein

If you post a larger picture showing where that is I can open mine up and look.

Also, any idea what caused this?

That would be great! Please see this photo, i have made a circle around the area;

Yes, i think im pretty sure this was caused by an overvoltage condition on the input. If i look at the error codes i get: 612, 614, and 618. All of these are related to the 1000 voltage range. Im very shocked that the resistors blew instead of what appears to be MOVs at the side.

This looks like a digital output judging by the opto-isolator i guess, perhaps a way to identify this is by following the traces and drawing a small schematic to judge the values from the rest of the circuit, you should start by removing both resistors. BTW i couldnt find a data sheet for U305 (likely an opto isolator)

where does the other end of L303 go?

The left end connects to the input jacks where as the other one goes into the shielded digitizer board to the ADC. However, it does seem weird why there would be optoislolators between the input and the ADC.

This looks like a digital output judging by the opto-isolator i guess, perhaps a way to identify this is by following the traces and drawing a small schematic to judge the values from the rest of the circuit, you should start by removing both resistors. BTW i couldnt find a data sheet for U305 (likely an opto isolator)

where does the other end of L303 go?

The left end connects to the input jacks where as the other one goes into the shielded digitizer board to the ADC. However, it does seem weird why there would be optoislolators between the input and the ADC.

This looks like a digital output judging by the opto-isolator i guess, perhaps a way to identify this is by following the traces and drawing a small schematic to judge the values from the rest of the circuit, you should start by removing both resistors. BTW i couldnt find a data sheet for U305 (likely an opto isolator)

where does the other end of L303 go?

The left end connects to the input jacks where as the other one goes into the shielded digitizer board to the ADC. However, it does seem weird why there would be optoislolators between the input and the ADC.

This looks like a digital output judging by the opto-isolator i guess, perhaps a way to identify this is by following the traces and drawing a small schematic to judge the values from the rest of the circuit, you should start by removing both resistors. BTW i couldnt find a data sheet for U305 (likely an opto isolator)

where does the other end of L303 go?

The left end connects to the input jacks where as the other one goes into the shielded digitizer board to the ADC. However, it does seem weird why there would be optoislolators between the input and the ADC.

which input jack? voltage or current?

Logged

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius, and a lot of courage to move in the opposite direction".- Albert Einstein

This looks like a digital output judging by the opto-isolator i guess, perhaps a way to identify this is by following the traces and drawing a small schematic to judge the values from the rest of the circuit, you should start by removing both resistors. BTW i couldnt find a data sheet for U305 (likely an opto isolator)

where does the other end of L303 go?

The left end connects to the input jacks where as the other one goes into the shielded digitizer board to the ADC. However, it does seem weird why there would be optoislolators between the input and the ADC.

This looks like a digital output judging by the opto-isolator i guess, perhaps a way to identify this is by following the traces and drawing a small schematic to judge the values from the rest of the circuit, you should start by removing both resistors. BTW i couldnt find a data sheet for U305 (likely an opto isolator)

where does the other end of L303 go?

The left end connects to the input jacks where as the other one goes into the shielded digitizer board to the ADC. However, it does seem weird why there would be optoislolators between the input and the ADC.

$90 is a great deal! I think I got mine for $300 after I found an issue and got a partial refund. Pretty amazing meter for less than the price of a Fluke 87V. Mine was working at the time. The issue turned out to be a dirty gang switch, which I just sprayed with contact cleaner. Of course I had to break the (current) cal seal to fix it.

A lot of powersupplies/transformers hum. I never noticed this meter making much noise, but the case is always on. Might not be a bad idea to at least change those big 1000uF caps (including the one closer to the front of the unit, right next to the transformer). They are all placed too close to heat sources for my liking.

Finally got the resistors and replaced them. Error codes still persist, but meter seems to be functioning correctly. I believe one of the self test circuitry might have been damaged during the over voltage condition. Ive found the following:

To check the operation of the internal current sources the meter outputs a constant current and measures the voltage across the precision caddock resistor divider. The unit has the following model USVD2-B10M-025-02:http://www.caddock.com/Online_catalog/Mrktg_Lit/TypeUSVD_HVD.pdf. These are 2 resistors, bonded as a ceramic sandwich. The larger 990K resistor which has the pins on the extreme end of the package seems to be spot on, but the smaller 100K resistor which has its pins on the inside measures 0.28Kohms . But as i cannot remove the resistor and both its pins are on the PCB whilst i measure it may just be wrong. Can someone measure the values of the resistor on pins 2 and 3 and report the value please?

Also the humming noise is coming from the transformers, it becomes a lot quieter once the meter heats up.

On yours are you able to see the capacitance and voltage ratings for C834 and C836, which are the two electrolytics next to the voltage regulators, but closest to the front of the unit? I cannot read those values without removing them, and I wanted to make sure I had the right values on hand before I did that.

Thanks.

Edit: with the shield removed I can read them as 100uF 25V

Also I see four more electrolytics under the shield. I wonder if replacing those would affect the calibration.

Ok i made a mistake, i can measure then to be around 231 ohms aswell! Phew it looks good then. But now, im even more confused with the error messages, the error message says that it tries to configure the 500nA current source to product a voltage across the caddock resistor, and because it has failed the 500nA source has failed. The other error i get is to do with the high voltage attenuator failure. Any idea how i can check these?

PS: Can you send me a photo of the meter without the shielding please.

It is easy to take off... Two screws, then slide the bottom piece towards the front back of the meter gently about 3-4mm. It has a couple of tabs that hold the two pieces together. At first I was worried it was being held on my thermal paste, but there is nothing contacting the shield.

Did you buy this meter broken, and thus have no clue what caused the problem, or did you break it on your end, and if so, how?